1. Field of the Invention
Embodiments of the invention relate to remote sensing of temperature and/or pressure. Particular embodiments relate to simultaneous optical sensing of temperature and pressure.
2. Discussion of Art
Optical distributed temperature sensors, commonly referred to as “DTS” systems, based on fiber optic sensing techniques are being used broadly in a number of applications and markets, including by way of example oil and gas wellbores, power distribution fault monitoring, and fire detection. These sensors use optical time delay reflectometry (OTDR), to measure temperature-encoded backscattered light signals propagating in the fiber to derive temperature at distinct positions all along the fiber. This allows a fully distributed temperature sensing architecture of which a number of applications particularly benefit of broad coverage such as pipelines, bridges, tunnels and oil and gas wells as examples
With DTS, high intensity pulsed laser energy is launched into a sensing fiber to stimulate nonlinear effects that cause light scattering. Optical DTS systems have been built based on Raman effects and other optical DTS systems have been built based on Brillouin effects, each producing both forward (Stokes) and backward (anti-Stokes) frequency-shifted signals or images in which their relative intensity ratio (in Raman systems) and/or combined Doppler shift (in Brillouin systems) is dependent on temperature. Raman effects and Brillouin effects are discussed in the paper, Daniele Inaudi and Branko Glisic, “Integration of distributed strain and temperature sensors in composite coiled tubing”, 2006 SPIE Smart Structures and Materials Conference, San Diego, Calif., Feb. 27 to Mar. 2, 2006, (Authors from SMARTEC SA, Via Pobiette 11, CH-6928 Manno, Switzerland, www.smartec.ch).
Of the nonlinear DTS sensors, the use of Raman type far exceeds that of the Brillouin type by virtue of the Raman effect, being photoelectric, is sensitive to temperature only, as compared to the Brillouin effect, being acoustic, that is sensitive to both temperature and strain. It is difficult to discriminate between these two parameters, thus the Brillouin effect requires complete isolation of fiber strain in order to support temperature measurement. Raman systems, being intensity-based, can operate on either single mode or multimode optical fibers. Telecom-grade graded index multimode fibers are used almost exclusively in Raman scattering applications, as the larger Raman cross-section and higher numerical aperture of such multimode fibers, compared to single mode fibers, yields greater intensity of backscattered signals, and greater collection of these signals
The Raman OTDR-based DTS operating on graded index multimode fibers is therefore the most common platform, and used almost exclusively across all applications. Oil and gas well monitoring systems have emerged as a very important application for these sensors, in which temperature and pressure information are primary physical parameters used by oil and gas producers to manage these wells. A common sensing architecture has a DTS sensing fiber installed directly along the well production tubing or casing, with a bottom-hole pressure gauge, either optical or electronic, connected by a transmission cable running as well to the surface instrumentation unit. The limitation of electronic gauge upper operating temperature has led to the proliferation of higher temperature rated optical pressure gauges in many such Oil and Gas well monitoring systems
While delivering suitable temperature/pressure data, both the all-optical and hybrid systems increase the complexity of the surface interrogation system, requiring separate interrogation instruments for DTS and pressure monitoring, but more importantly require multiple cables- either optical or hybrid, and associated multi-pin penetrations through packers and other well completion parts that add significantly to cost, complexity and difficulty installing and maintaining these systems.
Therefore, there has been for a long time a keen desire to operate both the DTS and pressure gauge along the same fiber to realize the benefits of a single fiber cable, single-pin downhole system in these wells. Unfortunately, known commercial optical pressure gauges suitable for downhole performance operate exclusively on single mode fibers that are incompatible with the common multimode Raman platform used extensively by this industry. Thus, there has been a longstanding need for a suitable pressure sensor that is operable on multimode optical fiber for seamless integration with the common Raman DTS platform. Such integration would provide a full well monitoring suite that couples DTS with bottom-hole pressure sensing, operating on a single cable/single penetration downhole system. Associated cost savings extend not only to the equipment, but also to reduction of complexity when installing a single cable system
There are not presently known to be any multimode optical pressure gauges in the commercial stream that are rated for the measurement performance and operating temperature/pressure conditions of subsurface oil and gas wells. Current optical pressure sensing solutions operate exclusively on single mode fibers due to the measurement principles employed, either phase gratings (Bragg gratings) or interferometric methods, which require single mode transmission to maintain signal information transmitted from the sensor to the surface instrument.